1. Field of Invention
This invention relates to improvements in packaged rooftop cooling units, and particularly to units that incorporate evaporative cooling to improve the efficiency, and reduce electrical demand of packaged rooftop cooling systems.
2. Description of Related Art
Most new low-rise non-residential buildings in the U.S. are cooled with packaged rooftop units (“RTU's”) that include one or more compressors, a condenser section that includes one or more air-cooled condensing coils and condenser fans, an evaporator coil, a supply blower, an intake location for outdoor ventilation air, optional exhaust air components, and controls. Most RTU's also include dampers that facilitate an “economizer” cycle to cool using only outdoor air when air temperature is favorable. These RTU components are packaged by manufacturers in similar configurations that, because they are air-cooled, are inefficient and result in a high electrical demand. This inefficiency is particularly significant in dry climate locations where rapid growth and focus on low construction costs have caused a high percentage of non-residential cooling systems to use traditional RTU's rather than more efficient “central systems” using chillers and evaporative cooling towers.
In climates where summer afternoon temperatures routinely reach 95° F. and higher, but with dry air such that wet bulb temperatures rarely exceed 70° F., traditional RTU cooling efficiencies can be increased by 20% to 25% using an evaporative condenser air pre-cooler (ECAP). A related patent (U.S. Pat. No. 6,574,975 B2) awarded to the applicant entitled “Dual Evaporative Pre-Cooling System and Method Therefor” describes improved ECAP systems that can be retrofitted to conventional RTU's to improve their efficiency by pre-cooling both condenser and ventilation air.
One reason that evaporatively-cooled RTU's are not widely available is the “high cost of maintenance” reputation of evaporative cooling systems. This reputation is due chiefly to problems associated with hardness minerals in water and biological growth. Both of these conditions can lead to deposits on surfaces and in heat exchangers that can significantly interfere with water distribution and operating performance. Hardness minerals are typically present when ground water is used for the water supply. Biological growth typically occurs in locations that remain continuously wet, as is true of many evaporative cooler reservoirs.
At least 10% of supply air delivered by traditional RTU's is typically outdoor air needed for building ventilation. In some cases, particularly for laboratory facilities, traditional RTU's deliver 100% outdoor air. In warm weather, cooling of ventilation air represents a significant fraction of the total cooling load. In very dry climates, ventilation air can be pre-cooled by the same direct evaporative process used in ECAP's, but in most applications an indirect process that adds no moisture to the ventilation air is preferred.
On the condenser side, traditional RTU's use high airflow rates to compensate for their air-cooled design. On the evaporator side, such RTU's typically send indoor air through a contorted path across both cooling and heating heat exchangers as the air is drawn through return ducts into the RTU, around several tight turns inside the unit, and back through supply ductwork. The added pressure drop associated with this complex path results in high fan energy consumption that penalizes the system all year, particularly in the constant-speed systems most common in traditional RTU's. To compensate for this load, a constant blower speed is set high enough for peak cooling load conditions which wastes blower energy at all other times.
In the prior art, several large RTU's have been marketed that take advantage of evaporative cooling to either pre-cool ventilation air or reduce condensing temperatures. For example, a product from Mammoth Industries condenses a hot gas refrigerant by circulating it through a piping array under a water spray, with air drawn upward through the array by fans to increase the evaporation rate and thereby reduce condensing temperatures. Another example is the “Oasis” unit produced by Des Champs Laboratories. The Oasis uses an evaporatively-cooled heat pipe system to pre-cool ventilation air.